The origins of the path are somewhat obscure, but researcher Tom Shieber has unearthed what is almost certainly the explanation. He noted that early baseball clubs often played on cricket grounds, where the two wickets were connected by a dirt path to ensure more reliable bounces. He speculates that early baseball clubs found that the path led to fewer passed balls and made it customary. Shieber cites a description that appeared in the New York Clipper in July 1860.

What this doesn't explain is why baseball fields commonly featured the "pitcher's path" well into the 20th century. Morris speculates -- and I agree -- that in the days of small grounds-keeping crews and limited technology, it was exceptionally difficult to keep a field in good shape; this is obvious if you look at photos of diamonds in the old days. With pitchers and catchers frequently trodding upon that stretch of ground between mound and plate, it was just easier to forget about grass and keep a dirt strip there instead.